1. Field of the Invention
The present invention relates to an ink jet printer, and particularly to an ink jet printer in which ultraviolet ray curable ink is used.
2. Description of Related Art
An ink jet printer is used to record an image on a recording medium by jetting ink from nozzles arranged on one surface of a recording head and making the ink collide with the recording medium. In this ink jet printer, an ink jet recording method of using ultraviolet ray curable ink has been known as a method of forming an image even on a recording medium having the property of a low ink absorption such as a resin film or the like (for example, disclosed in Published Japanese Patent Publication (Tokkaisyou) 60-132767). In the ink jet printer according to the ink jet recording method, ultraviolet curable ink, which includes a photochemical initiator having a predetermined sensitivity to ultraviolet rays, is used, the ink arriving at the recording medium is irradiated with the ultraviolet rays, and the ink is cured and fixed to the recording medium. In case of the ink jet printer using the ultraviolet curable ink, in the period of time from the arriving of the ink at the recording medium to the curing and fixing of the ink through the irradiation with the ultraviolet rays, a change in the image quality is caused by the enlarging of the diameter of dots of the ink arriving at the recording medium, the blurring of the recording medium with the ink between the dots and the permeation of the ink to the recording medium. Therefore, to form an image of the high quality by preventing the enlarging of the diameter of dots of the ink and the blurring of the recording medium with the ink, it is preferred that the period of time from the jetting of the ultraviolet ray curable ink to the irradiation of the ink with the ultraviolet rays is set to be as short as possible to cure and fix the ink in a short time.
Therefore, as shown in FIGS. 4 and 5, in the earlier developed ink jet printer, each of a plurality of recording heads having nozzles, from which the ultraviolet ray curable ink is jetted onto the recording medium, extends in the width direction of the recording medium, and the recording heads are arranged at predetermined intervals so as to make the longitudinal directions of the recording heads be parallel to one another, and a plurality of ultraviolet ray irradiating devices close to the recording heads respectively and having the same width and size as one another are arranged on the downstream side of the recording heads in the feeding direction of the recording medium respectively.
As described above, in the earlier development, each ultraviolet ray irradiating device emits a quantity of ultraviolet rays sufficient to almost perfectly cure the ink jetted onto the recording medium.
However, to obtain the quantity of ultraviolet rays required to almost perfectly cure the ink, large-sized ultraviolet ray irradiating devices are required, and the devices become large in size and complicated in structure. On the other hand, the change in the image quality caused by the enlarging of the diameter of dots of the ink, the blurring of the recording medium with the ink and the like occurs in dependent on the quantity of the ultraviolet rays, the irradiation timing of the ultraviolet rays, the wavelength of the ultraviolet rays, the intensity of the ultraviolet rays and the like. To form the image of the high quality while preventing the change in the image quality, the quantity in the ultraviolet rays required to cure the surfaces of the dots of the ink is sufficient when the ink is irradiated with the ultraviolet rays just after the jetting of the ink, and the quantity of the ultraviolet rays for curing the ink almost perfectly is not required. Therefore, the quantity of the ultraviolet rays is excessive in the earlier development to irradiate the ink with the ultraviolet rays. As a result, problems are arisen that the size of the device is enlarged, the cost of manufacturing the device is heightened, and electric power consumed to irradiate the ink with the ultraviolet rays is enlarged.
Further, generally, the ultraviolet ray irradiating devices and the recording heads are alternately arranged to place each ultraviolet ray irradiating device between two recording heads. Therefore, when the large-sized ultraviolet ray irradiating devices are used, the interval of two recording heads adjacent to each other is enlarged. As a result, because the irradiation timing of the ultraviolet rays is delayed, another problem is arisen that the enlarging of the diameter of dots of the ink, the blurring of the recording medium with the ink and the like easily occur.